1. Field of the Invention
The present invention relates to a conveyer for correcting a curl of a recording medium formed in an image forming apparatus such as a printer, a copier, a facsimile, or a printer, and to an image forming apparatus including the conveyer.
2. Description of the Related Art
By a method often adopted in known image forming apparatuses, a developing unit develops an electrostatic latent image that is formed on an image carrier into a toner image by using a laser or a light emitting diode (LED) array, for example, then the toner image is directly or indirectly transferred via an intermediate transfer belt, for example, to a recording medium, such as transfer paper, as an unfixed toner image. A fixing unit located downstream in a conveying direction is used to fix the unfixed toner image as a permanent image on the recording medium.
In this type of image forming apparatuses, the fixing unit is used to apply heat and pressure to the recording medium carrying the unfixed toner image, to fix the unfixed toner image onto the recording medium. However, upon fixing the unfixed toner image onto the recording medium, a so-called curl tends to be formed on the recording medium in the direction perpendicular to the conveying direction. Such a curl formed on the recording medium gives negative effects to conveyability of the recording medium conveyed thereafter, and to stackability of recording media in a recording medium ejecting unit such as an ejecting tray. Therefore, various countermeasures for correcting or reducing a curl formed on a recording medium have been invented and implemented in an image forming apparatus.
At the same time, if a curl correction is applied to a strong or a rigid recording medium, such as thick paper, that would not be curled so much in the fixing unit to start with, such a curl correction might have undesirable effects. For example, another type of curls might be formed (for example, a curl along other direction than the one corrected in the curl correcting unit), or the load of conveying the recording medium through the curl correcting means or unit could be increased.
In response to this issue, an invention for automatically omitting the application of a curl correction or adjusting the curl correction when the recording medium is a rigid one, such as thick paper, has been made. This kind of curl correcting unit is disclosed in Japanese Patent Application Laid-open No. 2006-168940.
The curl correcting unit disclosed in Japanese Patent Application Laid-open No. 2006-168940 includes a roller pair, located downstream of a fixing unit in the conveying direction, to correct a curl of a recording medium. One of the roller pair is rotatable around a roller axis of the other roller. Depending on the strength and the rigidity of a recording medium being conveyed, the rotatable roller rotates about the center of the other fixed roller, and co-operates with the form of a conveyance guiding member located downstream of the roller pair in the conveying direction, to automatically apply a curl correction based on the rigidity determined by the thickness of the recording medium, for example.
However, in a curl correcting unit such as one disclosed in Japanese Patent Application Laid-open No. 2006-168940, when the recording medium is thin, the front edge of the recording medium being conveyed is brought into contact with the conveyance guiding member, which is co-operating with and located downstream of the curl correcting roller pair in the conveying direction, at an almost obtuse angle, and as a result, the conveyance guiding member might obstruct conveyance of the recording medium. Consequently, so-called conveyance jamming might occur frequently. At the same time, when the recording medium is rigid like thick paper, one of the roller pair in the curl correcting unit rotates about the other. This might result in an insufficient force for conveying the recording medium, being unable to provide an appropriate conveying force. Moreover, because in such a curl correcting unit, it is difficult to convey a recording medium in a reverse direction. Therefore, the curl correcting unit would have difficulty in supporting a reversing operation of a recording medium, such as one performed upon executing double-sided image formation in which the recording medium is reversed and conveyed. In addition, because the roller pair is used as means for correcting a curl of the recording medium, the curvature radius of the recording medium is reduced while being nipped by the roller pair and being corrected of the curl. Therefore, in an application to a special recording medium, such as a release paper, the release member could be easily peeled off while passing through the roller pair. Therefore, such a technology has a problem in view of supportability for different types of paper strongly demanded by users, as well as in applications in double-sided image formation that is also highly demanded by users.